I have had several people ask me who is the main character of my thriller novel. But, the novel involves multiple characters working as a team who rise and fall as the need occurs. At first I thought it doesn't matter if there is a main character in a story about a small group working together. Do the X-men have a main character? Do the Avengers have a main character. How about the Walking Dead? But, my wife pointed out that yes, the Walking Dead do have a main character, it is Rick the sheriff. And thinking more on it I decided the Avengers have a main character also, it is Captain America. Even the X-men has one, the main character that continues even when they are not in the story isn't Dr Xavier, it is Wolverine. Her point is readers expect a main character because they want to know their origins, how their lives evolved to where they are in the story and how the other members feel about them. Too many 'leaders' become too confusing and readers simply lose caring for all the characters.
However, I also challenge the reader to name a single main character from Andromeda Strain or even Jurassic Park. A good editor friend told my my main character is the virus the team is trying to cure. Are the critique and my wife correct? Are main characters essential to both plot and as the anchor for point of view? Or is the foe the main character much like in Jaws or Moby Dick.